Everyone is going to be better at giving helpful critique by learning the art of constructive criticism.

That aside, even if your criticism wasn't tactful, Eowyn, it had content to it, and would be helpful to someone who was looking to improve his or her own work. And, since comments weren't turned off, you were invited to give comment. You shouldn't feel bad about doing what you were asked to do and attempting to be helpful to someone.

Furthermore, while praise is nice, it's meaningless if it's not honest.

This is probably a result of the prevailing high self-image, low self-improvement philosophy in public schools, in which children are encouraged to feel good about non-achievement.